In conventional devices for charging refrigerant into a refrigerant circuit, a supply pipe is provided in refrigerant piping, on the suction side of a compression mechanism in a refrigerant circuit, such that refrigerant can be charged into the refrigerant circuit by connecting a cylinder to the supply pipe, as disclosed in Patent Document 1. In such a charging device, the refrigerant flows through the supply pipe into the refrigerant circuit, to be charged into the latter, in accordance with the pressure difference between the refrigerant pressure in the cylinder and the pressure in the suction side of the compression mechanism.    Patent Document 1: JP 2001-74342 A
However, such charging devices have the following drawback. The refrigerant is supplied on account of the pressure difference between the refrigerant pressure in the cylinder and the pressure on the suction side of the compression mechanism, and thus the charging speed of the refrigerant varies depending on this pressure difference. As a result, the charging speed of the refrigerant decreases when the pressure in the cylinder drops on account of, for instance, a lower outdoor air temperature. This results in a longer charging time, which is problematic.